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Hi I received following error message:
• An error occurred while attempting to apply your configuration to Nagios Core. Monitoring engine configuration files have been rolled back to their last known good checkpoint.
Try Again
Error: Could not find any host matching 'nagios' (config file '/usr/local/nagios/etc/services/nagios.cfg', starting on line 336)
Error: Failed to expand host list 'nagios' for service 'Total Service Problems' (/usr/local/nagios/etc/services/nagios.cfg:336)

while trying to fix it using instructions from "article: 144 Nagios XI - Configuration Verification Failed:"

Error: Could not find any host matching 'af-admin01.af.local' (config file '/usr/local/nagios/etc/hostgroups.cfg', starting on line 70)
Error: Could not expand members specified in hostgroup (config file '/usr/local/nagios/etc/hostgroups.cfg', starting on line 70)

Nagios Core 4.2.4
Copyright (c) 2009-present Nagios Core Development Team and Community Contributors
Copyright (c) 1999-2009 Ethan Galstad
Last Modified: 12-07-2016
License: GPL

Website: https://www.nagios.org
Reading configuration data...
Read main config file okay...
Error: Could not find any host matching 'af-admin01.af.local' (config file '/usr/local/nagios/etc/hostgroups.cfg', starting on line 70)
Error: Could not expand members specified in hostgroup (config file '/usr/local/nagios/etc/hostgroups.cfg', starting on line 70)
Error processing object config files!


***> One or more problems was encountered while processing the config files...

Check your configuration file(s) to ensure that they contain valid
directives and data defintions. If you are upgrading from a previous
version of Nagios, you should be aware that some variables/definitions
may have been removed or modified in this version. Make sure to read
the HTML documentation regarding the config files, as well as the
'Whats New' section to find out what has changed.

Please Help resolving this issue.

As the error mentions, it would appear you have the host af-admin01.af.local defined in a hostgroup, in the hostgroups file. I would take a look here, and remove the host. When you remove a host, you also need to remove the relationships that go with it.

The issue is resolved

Awesome - going to lock this one up!